The present invention, in some embodiments thereof, relates to a logic design using memristors and, more particularly, but not exclusively, to a set of two logic blocks that allow full logic systems to be constructed therefrom.
As transistors continue to shrink in size, leakage current increases. In contemporary microprocessors, leakage has ceased being a negligible part of the power consumption. This fact is a key motivation for using non-volatile devices, such as memristors, in processing elements to reduce leakage current. Memristors are non-volatile circuit elements, predicted in 1971 by Leon Chua. In 2008, Hewlett Packard laboratories were the first to link resistive switching materials to the theory of memristors. As Chua suggested, we refer to all memristive devices as memristors, in the sense that all of the latter are two-terminal non-volatile memory devices based on resistance switching. Many types of materials can be associated with memristive behavior. These materials vary from molecular and ionic thin film semiconductors, through spin based and magnetic memristive systems, to phase change memories.
Switching in a memristive device, refers to the transition from one resistance state to another. Commonly, we distinguish between a high resistance state (HRS=“OFF”) and a low resistance state (LRS=“ON”). In the present application, LRS and HRS are considered, respectively, as logical ‘1’ and ‘0’. One prominent distinction of switching mechanisms is the classification of bipolar and unipolar switching. This classification is illustrated in FIGS. 1A-B, which illustrates curves I-V of (a) unipolar and (b) bipolar switching mechanisms of memristors. For bipolar switching, a transition from HRS→LRS (SET) occurs at a negative voltage (−VSET), while the transition from LRS→HRS (RESET) occurs at a positive voltage (VRESET). In the case of unipolar switching, a transition from HRS→LRS occurs when crossing a voltage threshold (VSET or −VSET). Typically, the current during the transition may be limited below a compliance current to avoid overloading the device. Resetting back to the OFF state happens at a voltage below VSET and above VRESET (or above −VSET and below −VRESET). A higher current is needed for switching to the OFF state. Unlike bipolar memristors, both transitions are independent of the voltage polarity.
The exact physical mechanism that promotes switching differs between devices and can be generally classified as thermal, electronic, or ionic. Bipolar switching is linked to cation/anion migration whereas unipolar switching is linked to the creation or dissolution of conducting filaments—this is often referred to as the fuse-antifuse mechanism. Both bipolar and unipolar memristors have already been incorporated into memory designs, and are also suggested to be used for performing logic operations. Some unipolar memristors have a high ROFF/RON ratio (HRS/LRS ratio), which makes them attractive candidates to perform logic operations due to a high noise margin. This increases motivation to choose unipolar memristors.